1. Field of the Invention
The present invention relates to a magnetic thin film that is used for a CPP-GMR (Current Perpendicular to the Plane Giant Magneto Resistance) element (hereinafter called a “CPP element”) which constitutes a thin-film magnetic head, and more particularly relates to the structure of a non-magnetic spacer layer.
2. Description of the Related Art
A CPP element is known as one of the magnetic field detecting elements which are used in a thin-film magnetic head. A CPP element has a pinned layer whose magnetization direction is fixed with respect to an external magnetic field, a free layer whose magnetization direction is changed in accordance with the external magnetic field, and a non-magnetic spacer layer that is sandwiched between the pinned layer and free layer (also called a “spacer layer”). In this specification, a stacked structure of layers that are comprised of a pinned layer, a non-magnetic spacer layer, and a free layer is called a “magnetic thin film.” A magnetic thin film is a central part of a CPP element for generating changes in magneto resistance by the GMR effect. A magnetic thin film forms a CPP element together with other metal layers. Sense current is configured to flow in a direction that is perpendicular to the film surfaces of the pinned layer, the non-magnetic spacer layer, and the free layer. A CPP element is sandwiched between a pair of shield layers. The shield layers also have the function of electrode layers for supplying sense current. Since the CPP element is physically connected with the shield layers, the CPP element has a high efficiency for heat radiation, and a large capacity for sense current. Further, the CPP element exhibits a larger electric resistance and a larger change in resistance in accordance with a decrease in cross section. Accordingly, the CPP element is more suitable for a narrow track width.
Techniques have been disclosed for improving the MR (Magneto Resistance) ratio of the CPP element. The specification of Japanese Patent No. 3293437 discloses a technique to vary the electric resistance of a non-magnetic spacer layer two-dimensionally in the film surface. The specification of Japanese Patent Laid-down Publication No. 2003-298143 and 2006-261306 discloses a non-magnetic spacer layer that is provided with a region that is made of oxides of Sn, Sb etc. and a region that is made of Cu, Ag etc. so that the electric resistance of the non-magnetic spacer layer is varied two-dimensionally in the film surface. According to these techniques, since most sense current flows through the region of smaller electric resistance, an effect can be achieved that is similar to the effect that would be obtained if the cross-section of the element was actually reduced. However, local migration is more likely to occur because current density is increased in the region of lower electric resistance because of the concentration of sense current in this region. For this reason, the CCP element of this type is disadvantageous in term of reliability.
As another technique for improving the MR ratio of a CPP element, Japanese Patent No. 3565268 discloses a non-magnetic spacer layer for which a semiconductor or half metal is used.